Polyurethanes which contain uretdione groups in the macromolecule are already known (Kunststoff Handbuch Volume VII, Polyurethane, published by Vieweg-Hochtlen, Carl-Hanser-Verlag Munich, 1966, pages 17, 37). It is also known that compounds which contain uretdione groups undergo openings of the uretdione ring at elevated temperatures to liberate the isocyanate groups (J. H. Saunders and K. C. Frishch "Polyurethanes, Chemistry and Technology", Part I, Interscience Publishers (1962) pages 113 et seq.).
The coating of metal articles with pulverulent coatings is also already known. Various polymers which are applied as powders to the metal article and form a protective layer on it when melted may be used as one-component systems which are either self cross-linking or cannot be cross-linked.
Two-component or multicomponent systems and their use as lacquer raw materials, e.g. mixtures of blocked polyisocyanates with polyesters which contain hyroxyl groups, etc. are also known in the art (German Pat. No. 946,143). These compounds have the advantage over products which contain free isocyanate groups that at low temperatures, e.g. room temperature, they remain practically unchanged even when stored for a considerable length of time in the presence of water and polyalcohols and only liberate the blocking agents at elevated temperatures, generally substantially above 100.degree. C, and then react with the polyfunctional reactants available, e.g. polyols, to form a cross-linked coating film.
These blocked isocyanates have the disadvantage that the blocking agent is liberated during the hardening reaction and given off to the surroundings. Some of the blocking agents are not physiologicaly harmless (phenols, mercaptans) and also have the added disadvantage of forming deposits in the cooler parts of a large stoving apparatus and therefore requiring substantial cleaning operations.
The polyaddition products with uretdione groups described in U.S. Pat. No. 3,248,370 and German Offenlegungsschriften Nos. 2,044,838 and 2,221,170 are also unsuitable for use as binders or binder components for thermosetting powder lacquers since they do not fulfill all the following basic requirements simultaneously:
a. the ability to be reduced to powders at temperatures below 30.degree. C PA1 b. softening or melting point between 30.degree. and 240.degree. C preferably 70.degree. to 180.degree. C; and PA1 c. (in combination with reactants which contain active hydrogen atoms) a hardening temperature at least 10.degree. to 40.degree. C above the sotening or melting point (of the combination) and preferably above 110.degree. C, in particular between 140.degree. and 220.degree. C. PA1 a. the reactants are reacted at an NCO/active hydrogen ratio of between 1:1 and 1:1.6; PA1 b. the compounds which contain hydrogen atoms capable of reacting with isocyanate groups are water or aliphatic, cycloaliphatic or araliphatic compounds with molecular weights of from 62 to 300 which contain a total of two hydroxyl groups, mixed with from 0 to 60% by weight, based on the total quantity of compounds which contain active hydrogen atoms, of aliphatic, cycloaliphatic or araliphatic compounds within a molecular weight range of from 32 to 300 which contain one hydroxyl group; and PA1 c. the proportions by weight of reactants are so chosen that the products of the process contain from 2 to 20% by weight of latent NCO groups and from 0 to 2 hydroxyl end-groups per macromolecule. PA1 1. Physiologically inert latent polyisocyanates from which no compounds are split off are obtained by a simple "in situ" process from diisocyanates and alcohols without changing the reaction vessel. PA1 2. With suitable choice and combination of starting compounds, the reactive systems according to the invention may be prepared not only in powder form, as is preferred according to the invention, i.e. in crystalline form, but also in a resinous or viscous form. PA1 3. The simple method of preparing the new polyuretdione polyurethanes as free flowing, physiologically inert reactive powders which are free from groups which are split off opens up new possibilities in the production of powder lacquers which may be stored. The polyuretdione-polyurethane cross-linking agents are compatible with a variety of numerous hard, linear or branched polyhydroxyl compounds and may be mixed with them in extruders without undergoing permature reactions. The extruders resins obtained are hard and brittle and may easily be ground down. Powders applied to sheets by the known electrostatic powder spray process flow at temperatures of about 80.degree. to 120.degree. C and may be stoved at temperatures of from 130.degree. to 220.degree. C, preferably from 150.degree. to 180.degree. C within 15 to 30 minutes. Matt to high gloss metal coatings which are weather resistant and elastic and have high impact strength are obtained after stoving. PA1 4. The reactive powders are stable in storage and not blocking even under extreme conditions (action of water, storage at 60.degree. C). They have no tendency to spontaneous ignition or dust explosions.
This invention points the way to new polyisocyanate polyaddition products which contain uretdione groups and which satisfy these conditions and are therefore valuable new binder components for thermosetting powder lacquers.